The pulp and paper industry is replete with instances where tank level and flow control systems are needed to ensure the proper control of, for example, material balances in the paper mill and to ensure against the environmental concerns of spills and inefficient blending of process materials. One such example of this need relates to the kraft papermaking operations around the world, and specifically to the byproduct operations in the kraft mill. In particular, the kraft papermaking process involves the production of a spent caustic solution (or weak black liquor) as a byproduct of the digester operation (where wood chips, for example, are broken down to cellulose fibers by the application of steam, pressure and chemicals). This spent caustic solution, after further concentration (to increase the percentage solids in the solution) and when stored in some tank arrangement, forms a precipitate called sulfate soap which, having a lower density than the weak black liquor, floats on the weak black liquor solution in the tank.
It is the flow and control of the sulfate soap and the weak black liquor which is the focus of the present invention. More specifically, the weak black liquor in a kraft papermill is usually processed further in the mill evaporators and used as a source of energy for the mill. If the mill evaporators are provided with a source of black liquor which has a high concentration of sulfate soap, the evaporator operation will be significantly less efficient than desired and in some instances may present serious environmental hazards.
The sulfate soap is also processed further, usually by acidulation, wherein the sulfate soap is blended with the appropriate amount of an acid, typically sulfuric acid, to produce a waste brine solution having a low pH, and a tall oil product. The presence of higher than optimal levels of weak black liquor in the sulfate soap may lead to higher costs for acidulation, unacidulated soap, and a host of environmental concerns related to the disposal of the higher levels of acid/brine solution.
In order to optimize the removal of the sulfate soap from the weak black liquor without removing an unnecessarily large amount of the weak black liquor along with the sulfate soap, tank level and flow controls for this portion of the papermill must be carefully controlled.
Referring to FIG. 1, representing a prior art storage tank arrangement for sulfate soap and weak black liquor, the mixture of weak black liquor and sulfate soap was sent to a tank where the sulfate soap would rise to the top of the mixture and the weak black liquor would become the lower layer in the tank. The creation of the mixture of weak black liquor and sulfate soap is the result of the reaction of the caustic pulping liquor solution with the wood chips in the digester. The separated sulfate soap phase must be first skimmed from the mixture (while still in one or more weak black liquor tanks) to remove the bulk of the precipitated sulfate soap. This skimming operation involves the removal of the precipitated sulfate soap and some of the weak black liquor. The "skimmed" portion of the weak black liquor tanks is then typically transferred to one or more sulfate soap storage tanks where the weak black liquor and the sulfate soap undergo further separation.
In reality, the separation of the components in the tank occurs in accordance with a density gradient with the lighter soap material rising to the top of the tank and the heavier weak black liquor material sinking to the bottom of the tank. It is normal for the sulfate soap in the top portion of the sulfate soap storage tank to contain a lower percentage of black liquor than the sulfate soap toward, for example, the middle of the tank. This separation of the weak black liquor and sulfate soap occurs because of the different densities of the two components of this mixture. In particular, the weak black liquor by itself, has a typical density range of 8.8 pounds per gallon to 9.3 pounds per gallon, while the sulfate soap, depending upon the amount of entrained air and weak black liquor, may have a typical density range of from 2 pounds per gallon to 8 pounds per gallon. Also, weak black liquor is a relatively free flowing, non-viscous material when compared to sulfate soap, which, depending upon the percentage of entrained weak black liquor, is a relatively viscous material. In fact, sulfate soap can typically have a viscosity range of approximately 2000 centipoise to 5000 centipoise, while weak black liquor, depending upon the amount of solids in the solution, has a typical viscosity range of 1 centipoise to 3 centipoise.
In the past, for example, and referring to FIG. 1, an operator would manually open a valve A at the bottom of the tank to remove the accumulated weak black liquor and either visually inspect the flow of material from the open valve, or approximate the amount of weak black liquor in the tank and keep the valve open for the correct amount of time to remove the weak black liquor. Other attempts at performing this operation included the use of temperature detection devices B and C, based on the differentials in the thermal conductivity characteristics of the weak black liquor and sulfate soap components. This procedure in particular has proven to be ineffective due to varying component composition and fouling of the temperature probes B and C in the weak black liquor and sulfate soap solutions, respectively, causing frequent cleanings and unreliable readings of tank levels.
Prior to the present invention, attempts to use viscosity detection as a means to determine the sulfate soap and weak black liquor interfaces in a tank failed because of the inability of such systems to accommodate the full range of flow conditions in the tank. Specifically, the former attempts failed to address the ever present process conditions relating to the high variability of process physical parameters and chemical composition.